There are commercially available high chromium stainless steels containing molybdenum and nickel, one such steel being Alloy No. 1 identified below.
It is known that small additions of platinum group metals to high chromium stainless steels have the effect of improving corrosion resistance.
It has also been shown that the combination of molybdenum and a platinum group metal in a high chromium stainless steel exhibits a synergistic beneficial effect on the corrosion resistance.
As far as applicant has been able to determine, the following are the closest compositions to the applicant's own invention that have been made or investigated and published in the past. Corrosion rates, in millimeters per annum (mm/a), were measured on samples of the respective stainless steels in boiling 10% by weight sulphuric acid, and the results are as follows:
______________________________________ Corrosion Alloy no. Alloy Composition rate (mm/a) ______________________________________ (1) Fe-29% Cr-4% Mo-2% Ni 0.23 (2) Fe-29% Cr-4% Mo-0.015% Ru 1580 (3) Fe-29% Cr-4% Mo-0.20% Ru 0.23 (4) Fe-29% Cr-4% Mo-0.1% Ni-0.01% Ru 1.04 ______________________________________
It has now been found that a further and additional synergistic effect is achieved if the chromium, molybdenum, nickel and/or copper, and a platinum group metal are used in combination in certain proportions.